Heat-cured furan binder systems are well known. Of particular interest are the so-called hot-box and warm-box binder systems. Hot-box furan binders typically comprise a furan, urea formaldehyde resin, and phenol formaldehyde resin. They are cured with an inorganic or organic salt at temperatures of 200° C. to 300° C. On the other hand, warm-box binder systems are typically comprised of a furan resin and furfuryl alcohol. They are cured with an inorganic or organic salt at temperatures of from about 100° C. to 300° C. Examples of warm-box binder systems are disclosed in U.S. Pat. Nos. 4,317,763, 4,451,577, and 4,383,098. These binders are preferred over hot-box binder for some applications because of their low formaldehyde content, low nitrogen content, higher tensile strength, lower binder usage level, availability of equipment, and the dimensional accuracy of the foundry shapes made with them.
Recently, improvements in warm-box binders have been made. Warm-box binder systems, which are nitrogen free, urea free, and essentially free of free formaldehyde, have been developed. Modifications of warm-box furan binders to increase their reactivity are also known. For instance, U.S. Pat. No. 5,607,986 discloses a binder comprising a phenol formaldehyde resin, modified furfuryl alcohol/formaldehyde resins, resorcinol pitch, and polyvinyl acetate to improve the reactivity of the warm box system. However, further improvements in these binders are still needed to improve their reactivity and the tensile strengths of cores made with them.
U.S. Pat. No. 6,391,942 relates furan no-bake binders comprising (a) a reactive furan resin, (b) furfuryl alcohol, (c) an activator selected from the group consisting of resorcinol, resorcinol pitch, and bisphenol A tar, (d) a bisphenol compound, (e) a polyol, and (f) a catalyst component comprising a catalytically effective amount of a halide of a transition metal. The patent discloses that the reactive furan may be a mixture of a conventional reactive furan and bis-hydroxymethylfuran.
All citations referred to in this patent application are expressly incorporated by reference.